Anti-reflection coating, optical member, exchange lens unit and imaging device

ABSTRACT

An anti-reflection coating comprising first to seventh layers formed on a substrate in this order, the first layer being an alumina-based layer, the seventh layer being a porous, silica-based layer, and each of the first to seventh layers having predetermined refractive index and optical thickness in a wavelength range of 400-700 nm.

FIELD OF THE INVENTION

The present invention relates to an anti-reflection coating suitable forexchange lens units for single-lens reflex cameras, etc., an opticalmember comprising such an anti-reflection coating, and an exchange lensunit and an imaging device comprising such an optical member.

BACKGROUND OF THE INVENTION

A high-performance, single-focus or zoom lens unit for single-lensreflex cameras, etc. generally has about 10-40 lenses in a lens barrel.Each lens has a laminated anti-reflection coating comprising pluralitiesof dielectric layers each having a different refractive index from thatof a lens substrate and a thickness of ½λ or ¼λ, wherein λ is a centerwavelength, to utilize interference effects. The anti-reflectioncharacteristics are important to a wide-angle lens, to which the angleof incident light is large particularly in its peripheral portion.

For instance, when an anti-reflection coating formed on each of 20lenses has reflectance of 0.5%, its transmittance is 0.995⁴⁰=0.818because the number of lens surfaces is 40, resulting in reflection lossof about 18%. Because reflection is superimposed in and between thelenses, large reflection loss of each lens provides the resultantphotographs with flare and ghost, as well as reduced contrast.Accordingly, an anti-reflection coating with small reflection lossshould be formed on a lens used in single-focus or zoom lens units.

In addition, blue tarnish, white tarnish, etc. may occur on the surfaceof a lens during a production process. The blue tarnish is a thin filmformed by basic components in optical glass dissolved into dew attachedto a surface of the optical glass left in the air, or water during agrinding step. The white tarnish is white blot generated by the chemicalreaction of components eluted from the glass. Accordingly, theanti-reflection coating formed on a glass lens should have a function toprevent tarnish.

JP 5-85778 A discloses an optical member comprising an anti-reflectioncoating having pluralities of dielectric layers on an optical substrate,the innermost layer being made of SiO_(x) (1≦x≦2) and having a thicknessnd of 0.25λ₀ or more, wherein λ₀ is a designed wavelength. However, thisanti-reflection coating has poor anti-reflection performance, and failsto fully prevent tarnish.

JP 10-20102 A discloses an anti-reflection coating comprising first toseventh layers formed in this order on a substrate having a refractiveindex of about 1.4-1.9 at a reference wavelength λ₀ arbitrarily designedin a visible wavelength range, the first to seventh layers meeting thefollowing conditions: 1.35≦n₁≦1.80, 1.90≦n₂≦2.50, 1.35≦n₃≦1.80,1.90≦n₄≦2.50, 1.35≦n₅≦1.80, 1.90≦n₆≦2.50, 1.35≦n₇≦1.50,0.0450λ₀≦n₁d₁≦0.2500λ₀, 0.0450λ₀≦n₂d₂≦0.1300λ₀≦0.0450λ₀≦n₃d₃≦0.1100λ₀,0.2100λ₀≦n₄d₄≦0.3000λ₀, 0.0450λ₀≦n₅d₅≦0.0750λ₀, 0.1000λ₀≦n₆d₆≦0.1600λ₀,0.2350λ₀≦n₇d₇≦0.2900λ₀, wherein n₁-n₇ are the refractive indices of thefirst to seventh layers, and n₁d₁-n₇d₇ are the optical thickness of thefirst to seventh layers. Although this anti-reflection coating has atarnish-preventing function because the first layer is made of alumina,its reflectance to visible light is as large as about 0.3% because theseventh layer is made of MgF₂.

JP 2001-100002 A discloses an anti-reflection coating having a 10-layerstructure comprising a MgF₂ layer, a ZrO₂/TiO₂ layer, an Al₂O₃ layer, aSiO₂ layer, a ZrO₂/TiO₂ layer, a SiO₂ layer, a ZrO₂/TiO₂ layer, a SiO₂layer, a ZrO₂/TiO₂ layer, and an Al₂O₃ layer in this order from thesurface, which has reflectance of about 0.1% at a visible wavelength of270 nm. JP2002-107506 A discloses an anti-reflection coating having a10-layer structure comprising a MgF₂ layer, a ZrO₂/TiO₂ layer, a SiO₂layer, an Al₂O₃ layer, a ZrO₂/TiO₂ layer, a SiO₂ layer, a ZrO₂/TiO₂layer, a SiO₂ layer, a ZrO₂/TiO₂ layer, and an Al₂O₃ layer in this orderfrom the surface, which has reflectance of about 0.1% at a visiblewavelength of 300 nm. However, because of the outermost MgF₂ layerhaving as high a refractive index as 1.38, both anti-reflection coatingsshould have as many as 10 layers for sufficient anti-reflectioncharacteristics.

JP 2005-352303 A discloses an anti-reflection coating comprisingpluralities of layers each having a physical thickness of 15-200 nm,which are formed on a substrate such that their refractive indicesdecrease gradually from the substrate side, the refractive indexdifference between adjacent layers and between the innermost layer andthe substrate being 0.02-0.2, and the outermost layer being a silicaaerogel layer. However, it has as large reflectance as more than 0.5% tovisible light near a wavelength of 400 nm, and fails to fully preventtarnish.

JP 2006-3562 A discloses an anti-reflection coating comprisingpluralities of layers each having a physical thickness of 15-200 nm,which are formed on a substrate such that their refractive indicesdecrease gradually from the substrate side, the refractive indexdifference between adjacent layers and between the innermost layer andthe substrate being 0.02-0.2, the innermost layer being an aluminalayer, and the outermost layer being a silica aerogel layer. Althoughthis anti-reflection coating has a tarnish-preventing function becausethe innermost layer is made of alumina, it does not have sufficientreflectance to visible light near a wavelength of 400 nm.

JP 2007-94150 A discloses an anti-reflection coating having 5 or 6layers, the outermost layer being a silica aerogel layer, which hasreflectance of 0.05% or less to visible light in a wavelength range of400-700 nm at an incident angle of 5°. However, because the innermostlayer is made of Ta₂O₅ or ZrO₂, tarnish cannot be fully suppressedunlike alumina.

OBJECT OF THE INVENTION

Accordingly, an object of the present invention is to provide ananti-reflection coating having excellent anti-reflection characteristicswithout suffering tarnish, an optical member having such ananti-reflection coating, and an exchange lens unit and an imaging devicecomprising such an optical member.

DISCLOSURE OF THE INVENTION

Thus, the first anti-reflection coating of the present inventioncomprises first to seventh layers formed on a substrate in this order,the first layer being an alumina-based layer, the seventh layer being aporous, silica-based layer, and in a wavelength range of 400-700 nm,

-   -   the substrate having a refractive index of 1.60-1.95,    -   the first layer having an optical thickness of 37.5-112.5 nm,    -   the second layer having a refractive index of 1.95-2.25 and an        optical thickness of 35.5-60.0 nm,    -   the third layer having a refractive index of 1.35-1.50 and an        optical thickness of 24.5-41.5 nm,    -   the fourth layer having a refractive index of 1.95-2.25 and an        optical thickness of 210.5-250.0 nm,    -   the fifth layer having a refractive index of 1.35-1.50 and an        optical thickness of 12.5-32.5 nm,    -   the sixth layer having a refractive index of 1.95-2.25 and an        optical thickness of 27.5-45.0 nm, and    -   the seventh layer having an optical thickness of 108.0-138.0 nm.

In the first anti-reflection coating, the second layer, the fourth layerand the sixth layer are preferably made of at least one materialselected from the group consisting of Ta₂O₅, ZrO₂, HfO₂, CeO₂, SnO₂,In₂O₃ and ZnO, and the third layer and the fifth layer are preferablymade of MgF₂ and/or SiO₂. At least one material selected from the groupconsisting of TiO₂, Nb₂O₅, Y₂O₃ and Pr₆O₁₁ may be added to the secondlayer, the fourth layer and the sixth layer. Al₂O₃ may be added to thethird layer and the fifth layer.

In the first anti-reflection coating, the first layer preferably has arefractive index of 1.59-1.69, and the seventh layer preferably has arefractive index of 1.25-1.30.

The first anti-reflection coating preferably has reflectance of 0.5% orless to incident light at 0° in a wavelength range of 450-600 nm, andreflectance of 1.0% or less to incident light at 30° in a wavelengthrange of 400-650 nm.

The second anti-reflection coating of the present invention comprisesfirst to seventh layers formed on a substrate in this order, the firstlayer being an alumina-based layer, the seventh layer being a porous,silica-based layer, and in a wavelength range of 400-700 nm,

-   -   the substrate having a refractive index of 1.50-1.70,    -   the first layer having an optical thickness of 24.5-200.0 nm,    -   the second layer having a refractive index of 1.93-2.25 and an        optical thickness of 24.5-50.5 nm,    -   the third layer having a refractive index of 1.33-1.50 and an        optical thickness of 14.0-30.0 nm,    -   the fourth layer having a refractive index of 2.00-2.16 and an        optical thickness of 131.5-200.5 nm,    -   the fifth layer having a refractive index of 1.33-1.50 and an        optical thickness of 20.0-31.5 nm,    -   the sixth layer having a refractive index of 2.04-2.17 and an        optical thickness of 50.0-62.5 nm, and    -   the seventh layer having an optical thickness of 122.5-142.5 nm.

In the second anti-reflection coating, the second layer, the fourthlayer and the sixth layer are preferably made of at least one materialselected from the group consisting of Ta₂O₅, ZrO₂, HfO₂, CeO₂, SnO₂,In₂O₃ and ZnO, and the third layer and the fifth layer are preferablymade of MgF₂ and/or SiO₂. At least one material selected from the groupconsisting of TiO₂, Nb₂O₅, Y₂O₃ and Pr₆O₁₁ may be added to the secondlayer, the fourth layer and the sixth layer. Al₂O₃ may be added to thethird layer and the fifth layer.

In the second anti-reflection coating, the first layer preferably has arefractive index of 1.57-1.72, and the seventh layer preferably has arefractive index of 1.23-1.32.

The second anti-reflection coating has reflectance of 0.3% or less toincident light at 0° in a wavelength range of 450-600 nm.

In the first and second anti-reflection coatings, the seventh layer ispreferably a silica aerogel layer.

In the first and second anti-reflection coatings, a fluororesin layer asthick as 0.4-100 nm having water repellency or water/oil repellency ispreferably formed on the seventh layer.

In the first and second anti-reflection coatings, the first to sixthlayers are preferably formed by a physical vapor deposition method, andthe seventh layer is preferably formed by a wet method. The physicalvapor deposition method is preferably a vacuum vapor deposition method,and the wet method is preferably a sol-gel method.

The optical member of the present invention comprises the aboveanti-reflection coating. The exchange lens unit of the present inventioncomprises the above optical member. The imaging device of the presentinvention comprises the above optical member.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing the anti-reflection coatingformed on a substrate according to one embodiment of the presentinvention.

FIG. 2 is a cross-sectional view showing the anti-reflection coatingformed on a substrate according to another embodiment of the presentinvention.

FIG. 3 is a graph showing the spectral reflectance of theanti-reflection coating of Example 1.

FIG. 4 is a graph showing the spectral reflectance of theanti-reflection coating of Example 2.

FIG. 5 is a graph showing the spectral reflectance of theanti-reflection coating of Example 3.

FIG. 6 is a graph showing the spectral reflectance of theanti-reflection coating of Example 4.

FIG. 7 is a graph showing the spectral reflectance of theanti-reflection coating of Example 5.

FIG. 8 is a graph showing the spectral reflectance of theanti-reflection coating of Comparative Example 1.

FIG. 9 is a graph showing the spectral reflectance of theanti-reflection coating of Example 7.

FIG. 10 is a graph showing the spectral reflectance of theanti-reflection coating of Example 8.

FIG. 11 is a graph showing the spectral reflectance of theanti-reflection coating of Example 9.

FIG. 12 is a graph showing the spectral reflectance of theanti-reflection coating of Comparative Example 2.

FIG. 13 is a schematic view showing an example of apparatuses forforming the anti-reflection coating.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[1] First Anti-Reflection Coating

(1) Layer Structure of Anti-Reflection Coating

The first anti-reflection coating 1 according to one embodiment of thepresent invention comprises, as shown in FIG. 1, first to seventh layers11-17 formed on a substrate 3 in this order. The first layer 11 is basedon alumina, and the seventh layer 17 is a porous, silica-based layer. Ina wavelength range of 400-700 nm,

-   the substrate 3 has a refractive index of 1.60-1.95,-   the first layer 11 has an optical thickness of 37.5-112.5 nm,-   the second layer 12 has a refractive index of 1.95-2.25 and an    optical thickness of 35.5-60.0 nm,-   the third layer 13 has a refractive index of 1.35-1.50 and an    optical thickness of 24.5-41.5 nm,-   the fourth layer 14 has a refractive index of 1.95-2.25 and an    optical thickness of 210.5-250.0 nm,-   the fifth layer 15 has a refractive index of 1.35-1.50 and an    optical thickness of 12.5-32.5 nm,-   the sixth layer 16 has a refractive index of 1.95-2.25 and an    optical thickness of 27.5-45.0 nm, and-   the seventh layer 17 has an optical thickness of 108.0-138.0 nm,-   wherein the optical thickness=refractive index x physical thickness.

The refractive index of the alumina-based first layer 11 is preferably1.59-1.69, more preferably 1.63-1.67. The first layer 11 preferably hasan optical thickness of 60.0-100.0 nm. Alumina has particularly highadhesion to a glass substrate, high transmittance in a wide wavelengthrange, high hardness, excellent wear resistance, and good costefficiency. Because alumina has excellent shielding to water vapor, thealumina-based first layer 11 can prevent tarnish on the surface of thesubstrate 3.

The seventh layer 17 is preferably made of a silica aerogel to provide alow refractive index and an excellent anti-reflection function. Therefractive index of the seventh layer 17 is preferably 1.25-1.30, morepreferably 1.26-1.29. The porous seventh layer 17 preferably has a porediameter of 0.005-0.2 μm, a porosity of 20-60%, and an optical thicknessof 115.0-138.0 nm. The seventh layer 17 made of a silica aerogel may behydrophobized. The hydrophobized silica aerogel layer has excellentwater resistance and durability.

The second layer 12 preferably has an optical thickness of 36.0-52.0 nmand a refractive index of 2.02-2.20. The third layer 13 preferably hasan optical thickness of 27.0-37.0 nm and a refractive index of1.36-1.45. The fourth layer 14 preferably has an optical thickness of210.0-247.0 nm and a refractive index of 2.00-2.15. The fifth layer 15preferably has an optical thickness of 17.0-25.0 nm and a refractiveindex of 1.36-1.45. The sixth layer 16 preferably has an opticalthickness of 32.0-43.0 nm and a refractive index of 2.00-2.20.

(2) Materials

The first layer 11 is based on alumina (aluminum oxide), whose purity ispreferably 99% or more by mass. Any of the second layer 12, the fourthlayer 14 and the sixth layer 16 is preferably made of at least onematerial selected from the group consisting of Ta₂O₅, ZrO₂, HfO₂, CeO₂,SnO₂, In₂O₃ and ZnO. Any of the third layer 13 and the fifth layer 15 ispreferably made of MgF₂ and/or SiO₂. At least one material selected fromthe group consisting of TiO₂, Nb₂O₅, Y₂O₃ and Pr₆O₁₁ may be added to thesecond layer 12, the fourth layer 14 and the sixth layer 16. Al₂O₃ maybe added to the third layer 13 and the fifth layer 15. The seventh layer17 is based on silica, which is preferably silica aerogel.

(3) Production Method

[a] Formation Method of First to Sixth Layers

The first layer 11 to the sixth layer 16 are preferably formed by aphysical vapor deposition method such as a vacuum vapor depositionmethod, a sputtering method, etc., particularly a vacuum vapordeposition method from the aspect of a production cost and precision.

The vacuum vapor deposition method may be a resistor-heating type, anelectron-beam type, etc. The electron-beam vacuum vapor depositionmethod will be explained below referring to a vacuum vapor depositionapparatus 30 shown in FIG. 13. The vacuum vapor deposition apparatus 30comprises, in a vacuum chamber 31, a rotatable rack 32 for carryingpluralities of lenses on its inner surface, a vapor source 33 comprisinga crucible 36 containing an evaporating material, an electron beamirradiator 38, a heater 39, and a vacuum pump connector 35 connected toa vacuum pump 40. To form the first to sixth layers in the firstanti-reflection coating 1 on each lens 100, the lens 100 is attached tothe rotatable rack 32 such that its surface is oriented to the vaporsource 33, and the evaporating material 37 is placed in the crucible 36.After the vacuum chamber 31 is evacuated by the vacuum pump 40 connectedto the vacuum pump connector 35, the lens 100 is heated by the heater39. While rotating the rotatable rack 32 by a shaft 34, electron beamsare irradiated from the electron beam irradiator 38 to the evaporatingmaterial 37 to heat it. The vaporized material 37 is deposited on thelens 100, so that each layer in the first anti-reflection coating 1 isformed on the lens 100.

In the vacuum vapor deposition method, the initial degree of vacuum ispreferably 1.0×10⁻⁵ Torr to 1.0×10⁻⁶ Torr. When the degree of vacuum isless than 1.0×10⁻⁵ Torr, insufficient vapor deposition occurs. When thedegree of vacuum is more than 1.0×10⁻⁶ Torr, it takes too much time forvapor deposition, resulting in poor production efficiency. To increasethe precision of the formed layers, it is preferable to heat the lensduring vapor deposition. The lens temperature during vapor depositionmay be properly determined based on the heat resistance of the lens andthe vapor deposition speed, but it is preferably 60-250° C.

[b] Formation Method of Seventh Layer

(i) Preparation of Organically Modified Silica Dispersion

(i-1) Formation of Wet Gel

The wet gel is formed by dissolving a silica-skeleton-forming compoundand a catalyst in a solvent, causing the hydrolysis and polymerizationof the silica-skeleton-forming compound, and then conducting aging.

(a) Silica-Skeleton-Forming Compound

(a-1) Saturated alkoxysilane and silsesquioxane

Silica sol and gel are formed by the hydrolysis and polymerization ofalkoxysilane and/or silsesquioxane. The saturated alkoxysilane may be amonomer or an oligomer. The saturated alkoxysilane monomer preferablyhas 3 or more alkoxy groups. Using a saturated alkoxysilane having 3 ormore alkoxy groups as a silica-skeleton-forming compound,anti-reflection coatings with excellent uniformity can be obtained.Specific examples of the saturated alkoxysilane monomers includemethyltrimethoxysilane, methyltriethoxysilane, phenyltriethoxysilane,tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane,tetrabutoxysilane, diethoxydimethoxysilane, dimethyldimethoxysilane, anddimethyldiethoxysilane. The saturated alkoxysilane oligomers arepreferably polycondensates of these monomers. The saturated alkoxysilaneoligomers can be obtained by the hydrolysis and polymerization of themonomers.

The use of a saturated silsesquioxane as a silica-skeleton-formingcompound can also provide an anti-reflection coating with excellentuniformity. The saturated silsesquioxane is a general name ofpolysiloxanes in the form of network, which have structural unitsrepresented by the general formula: RSiO_(1.5), wherein R represents anorganic functional group. R may be, for instance, a linear or branchedalkyl group having 1-6 carbon atoms, a phenyl group, or an alkoxy group(a methoxy group, an ethoxy group, etc.). It is known that thesilsesquioxane has various structures such as a ladder structure, a cagestructure, etc. It has excellent weather resistance, transparency andhardness, suitable as a silica-skeleton-forming compound for the silicaaerogel.

(a-2) Unsaturated alkoxysilane monomer and silsesquioxane

An unsaturated monomer or oligomer of alkoxysilane or silsesquioxanehaving an ultraviolet-polymerizable unsaturated group may be used as asilica-skeleton-forming compound. Using the silica-skeleton-formingcompound having an unsaturated group, a silica aerogel coating withexcellent toughness can be obtained even when a small amount of a binderis added. The unsaturated alkoxysilane monomer has an organic grouphaving at least one double or triple bond (hereinafter referred to as“unsaturated group”), and an alkoxy group. The unsaturated group has2-10 carbon atoms, preferably 2-4 carbon atoms.

The preferred unsaturated alkoxysilane monomer is represented by thefollowing general formula (1);

R^(a)Si(OR^(b))₃   (1),

wherein R^(a) represents an organic group having an unsaturated bond and2-10 carbon atoms, and R^(b)O represents an alkoxy group having 1-4carbon atoms.

The unsaturated group R^(a) is an organic group having at least oneultraviolet-polymerizable unsaturated bond, which may have asubstituting group such as a methyl group, an ethyl group, etc. Specificexamples of the unsaturated group R^(a) include a vinyl group, an allylgroup, a methacryloxy group, an aminopropyl group, a glycidoxy group, analkenyl group and a propargyl group. R^(b) is an organic group, whichmay be the same as or different from R^(a). Specific examples of thealkoxy group R^(b)O include a methoxy group, an ethoxy group, a propoxygroup, a butoxy group, an isopropoxy group and an s-butoxy group.

Specific examples of the unsaturated alkoxysilane monomers includetrimethoxyvinylsilane, triethoxyvinylsilane, allyltrimethoxysilane,allyltriethoxysilane, tributoxyvinylsilane, tripropoxyvinylsilane,allyltributoxysilane, allyltripropoxysilane, dimethoxydivinylsilane,diallyldimethoxysilane, diethoxydivinylsilane, diallyldiethoxysilane,trimethoxybutenylsilane, triethoxybutenylsilane,dibutenyldimethoxysilane, and di(3-butenyl)diethoxysilane.

An oligomer of the unsaturated alkoxysilane may be used as asilica-skeleton-forming compound. The unsaturated alkoxysilane oligomeralso has at least one unsaturated group and at least one alkoxy group.The unsaturated alkoxysilane oligomer is preferably represented by thefollowing general formula (2);

Si_(m)O_(m−1)R^(a) _(2m+2-x)OR^(b) _(x)   (2),

wherein R^(a) represents an organic group having an unsaturated bond and2-10 carbon atoms, R^(b)O represents an alkoxy group having 1-4 carbonatoms, m represents an integer of 2-5, and x represents an integer of4-7. Preferred examples of the unsaturated groups R^(a) and the alkoxygroups R^(b)O are the same as those in the above alkoxysilane monomers.

The number m of condensation is preferably 2-3. An oligomer whose numberm of condensation is 5 or less can be easily obtained by thepolymerization of the monomer using an acidic catalyst as describedbelow. The number x of the alkoxy group is preferably 3-5. When thenumber x of the alkoxy group is less than 3, the hydrolysis andpolycondensation of the alkoxysilane does not sufficiently proceed,making three-dimensional cross-linking difficult to occur, therebymaking the formation of a wet gel too difficult. When the number x ofthe alkoxy group is more than 5, the percentage of the unsaturated groupis too small, resulting in insufficient increase in mechanical strengthby the polymerization. Specific examples of the unsaturated alkoxysilaneoligomers include disilanes, trisilanes and tetrasilanes obtained by thecondensation of the above unsaturated alkoxysilane monomers.

(b) Solvent

The solvent is preferably composed of water and alcohol. The alcohol ispreferably methanol, ethanol, n-propyl alcohol, and isopropyl alcohol,particularly ethanol. How active the hydrolysis and polycondensationreaction are depends on a molar ratio of the monomer and/or oligomer ofalkoxysilane or silsesquioxane (silica-skeleton-forming compound) towater. Though the water/alcohol molar ratio does not directly affect thehydrolysis and polycondensation reaction, it is preferably substantially0.1-2. When the water/alcohol molar ratio is more than 2, the hydrolysisproceeds too quickly. When the water/alcohol molar ratio is less than0.1, the hydrolysis of the silica-skeleton-forming compound does notsufficiently occur.

(c) Catalyst

A catalyst for the hydrolysis reaction is added to an aqueous solutionof the silica-skeleton-forming compound. The catalyst may be acidic orbasic. For instance, an efficient hydrolysis can be proceeded bycondensing the silica-skeleton-forming compound monomer to an oligomerin an aqueous solution containing an acidic catalyst, and polymerizingthe oligomer in a solution containing a basic catalyst. Specificexamples of the acidic catalysts include hydrochloric acid, nitric acidand acetic acid. Specific examples of the basic catalysts includeammonia, amines, NaOH and KOH. Preferred examples of the amines includealcohol amines, and alkyl amines (methylamine, dimethylamine,trimethylamine, n-butylamine, and n-propylamine, etc.).

(d) Formulation

The silica-skeleton-forming compound is preferably dissolved in thesolvent, such that a molar ratio of the solvent to alkoxysilane is3-100. When the molar ratio is less than 3, the degree of polymerizationof the alkoxysilane is too high. When the molar ratio exceeds 100, thedegree of polymerization of the alkoxysilane becomes too low. Acatalyst/alkoxysilane molar ratio is preferably 1×10⁻⁷ to 1×10⁻¹, morepreferably 1×10⁻² to 1×10⁻¹. When the molar ratio is less than 1×10⁻⁷,the hydrolysis of the alkoxysilane does not occur sufficiently. Even ata molar ratio of more than 1×10⁻¹, increased catalytic effects cannot beobtained. A water/alkoxysilane molar ratio is preferably 0.5-20, morepreferably 5-10.

(e) Aging

A solution containing the silica-skeleton-forming compound condensed byhydrolysis is left to stand or slowly stirred for aging at 25-90° C. forabout 20-60 hours. Gelation proceeds by aging, to form a wet gelcontaining silicon oxide.

(i-2) Substitution of Dispersing Medium

A dispersing medium of the wet gel influences a surface tension and/or acontact angle of a solid phase to a liquid phase, which accelerate orretard aging, an extent of surface modification in the organicmodification step, and an evaporation rate of the dispersing medium inthe later-described coating step. The dispersing medium contained in thegel can be substituted by another dispersing medium by repeating anoperation of pouring another dispersing medium into the gel, vibratingthe gel and conducting decantation. The substitution of the dispersingmedium may be conducted before or after an organic modificationreaction, though it is preferably conducted before the organicmodification reaction to reduce the number of steps.

Specific examples of the substituting dispersing media include ethanol,methanol, propanol, butanol, pentane, hexane, heptane, cyclohexane,toluene, acetonitrile, acetone, dioxane, methyl isobutyl ketone,propylene glycol monomethyl ether, ethylene glycol monomethyl ether, andethyl acetate. These dispersing media may be used alone or incombination.

The preferred substituting dispersing media are ketones. Substitutionwith a ketone solvent before the later-described ultrasonic treatmentstep makes it possible to obtain a well-dispersible, organicallymodified, silica-containing sol. Because the ketone solvent hasexcellent affinity for silica (silicon oxide) and organically modifiedsilica, organically modified silica is well dispersed in the ketonesolvent. The preferred ketone solvent has a boiling point of 60° C. orhigher. Ketones having boiling points of lower than 60° C. areevaporated too much in the later-described ultrasonic irradiation step.For instance, acetone used as a dispersing medium is much evaporatedduring the ultrasonic irradiation, resulting in difficulty incontrolling the concentration of the dispersion. Acetone is quicklyevaporated in the coating step, too, failing to keep a sufficientcoating time. It is further known that acetone is harmful to humans,unpreferable for the health of an operator.

Particularly preferred as the ketone solvent is unsymmetrical ketonehaving different groups on both sides of a carbonyl group. Becausenonsymmetrical ketone has a large polarity, it has excellent affinityparticularly for silica and organically modified silica. The organicallymodified silica preferably has a particle size of 200 nm or less in thedispersion. When the particle size of the organically modified silica ismore than 200 nm, it is difficult to form a silica aerogel coatinghaving a substantially smooth surface.

The ketone may have an alkyl or aryl group. The preferred alkyl grouphas about 1-5 carbon atoms. Specific examples of the ketone solventsinclude methyl isobutyl ketone, ethyl isobutyl ketone, and methyl ethylketone.

(i-3) Organic Modification

An organic-modifying agent solution is added to the wet gel, so thathydrophilic groups such as a hydroxyl group, etc. at the end of siliconoxide constituting the wet gel are substituted by hydrophobic organicgroups.

(a) Organic-Modifying Agents

(a-1) Saturated Organic-Modifying Agents

The preferred saturated organic-modifying agent is any of compoundsrepresented by the following formulae (3)-(8);

R^(c) _(p)SiCl_(q)   (3),

R^(c) ₃SiNHSiR^(c) ₃   (4),

R^(c) _(p)Si(OH)_(q)   (5),

R^(c) ₃SiOSiR^(c) ₃   (6),

R^(c) _(p)Si(OR^(b))_(q)   (7), and

R^(c) _(p)Si(OCOCH₃)_(q)   (8),

wherein p represents an integer of 1-3, q represents an integer of 1-3satisfying the condition of q=4−p, R^(b)O represents an alkoxy grouphaving 1-4 carbon atoms, and R^(c) represents hydrogen, a substituted orunsubstituted alkyl group having 1-18 carbon atoms, or a substituted orunsubstituted aryl group having 5-18 carbon atoms, or a mixture thereof.

Specific examples of the saturated organic-modifying agents includetriethylchlorosilane, trimethylchlorosilane, diethyldichlorosilane,dimethyldichlorosilane, acetoxytrimethylsilane, acetoxysilane,diacetoxydimethylsilane, methyltriacetoxysilane, phenyltriacetoxysilane,diphenyldiacetoxysilane, trimethylethoxysilane, trimethylmethoxysilane,2-trimethylsiloxy pent-2-en-4-one, N-(trimethylsilyl)acetamide,2-(trimethylsilyl)acetate, N-(trimethylsilyl)imidazole, trimethylsilylpropiolate, nonamethyltrisilazane, hexamethyldisilazane,hexamethyldisiloxane, trimethylsilanol, triethylsilanol,triphenylsilanol, t-butyldimethylsilanol, diphenylsilanediol, etc.

(a-2) Unsaturated, Organic-Modifying Agents

Using the unsaturated organic-modifying agent, a silica aerogel coatingwith excellent toughness can be obtained even when a small amount of abinder is added. Preferred examples of the unsaturated organic-modifyingagents are represented by the following formulae (9)-(14);

R^(d) _(p)SiCl_(q)   (9),

R^(d) ₃SiNHSiR^(d) ₃   (10),

R^(d) _(p)Si(OH)_(q)   (11)

R^(d) ₃SiOSiR^(d) ₃   (12),

R^(d) _(p)Si(OR^(d))_(q)   (13),

R^(d) _(p)Si(OCOCH₃)_(q)   (14),

wherein p represents an integer of 1-3, q represents an integer of 1-3meeting the condition of q=4−p, and R^(d) represents an organic grouphaving an ultraviolet-polymerizable, unsaturated bond and 2-10 carbonatoms. The unsaturated group R^(d) may have a methyl group, an ethylgroup, etc. Examples of the unsaturated group R^(d) include a vinylgroup, an allyl group, a methacryloxy group, an aminopropyl group, aglycidoxy group, an alkenyl group, and a propargyl group. Theunsaturated organic-modifying agent may be used alone or in combination.The unsaturated organic-modifying agent may be combined with thesaturated organic-modifying agent.

The unsaturated organic-modifying agent is preferably unsaturatedchlorosilane, more preferably unsaturated monochlorosilane having threeunsaturated groups. Specific examples of the unsaturatedorganic-modifying agents include triallylchlorosilane,diallyldichlorosilane, triacetoxyallylsilane, diacetoxydiallylsilane,trichlorovinylsilane, dichlorodivinylsilane, triacetoxyvinylsilane,diacetoxydiallylsilane, trimethoxybutenylsilane, triethoxybutenylsilane,dibutenyldimethoxysilane, dibutenyldiethoxysilane, etc.

(b) Organic Modification Reaction

The organic-modifying agent is preferably dissolved in a solvent such ashydrocarbons such as pentane, hexane, cyclohexane, heptane, etc.;ketones such as acetone, etc.; aromatic compounds such as benzene,toluene, etc. The organic modification is preferably conducted at 10-40°C., although variable depending on the type and concentration of theorganic-modifying agent. When the organic-modifying temperature is lowerthan 10° C., the organic-modifying agent does not easily react withsilicon oxide. When it is higher than 40° C., the organic-modifyingagent easily reacts with other substances than silicon oxide. Thesolution is preferably stirred to avoid a distribution in temperatureand concentration in the solution during the reaction. For instance,when the organic-modifying agent solution is a solution oftriethylchlorosilane in hexane, holding at 10-40° C. for about 20-40hours (for instance, 30 hours) sufficiently turns a silanol group to asilyl group.

(i-4) Ultrasonic Treatment

The ultrasonic treatment turns the organically modified silica gel orsol to be suitable for coating. In the case of the organically modifiedsilica gel, the ultrasonic treatment dissociates a gel coagulated by anelectric force or a van der Waals force, and destroys covalent bonds ofsilicon to oxygen, resulting in a dispersed gel. In the case of the sol,too, the ultrasonic treatment reduces the agglomeration of colloidparticles. The ultrasonic treatment can be conducted in a dispersingapparatus using an ultrasonic vibrator. An ultrasonic radiationfrequency is preferably 10-30 kHz, and an output is preferably 300-900W.

The ultrasonic treatment time is preferably 5-120 minutes. Longerultrasonic irradiation results in finer pulverization of clusters of thegel or the sol, resulting in less agglomeration. Accordingly, colloidparticles of organically modified silicon oxide are almost in a singledispersion state in the silica-containing sol obtained by the ultrasonictreatment. When the ultrasonic treatment time is shorter than 5 minutes,the colloid particles are not sufficiently dissociated. Even if theultrasonic treatment time were longer than 120 minutes, the dissociationof the colloid particles of the organically modified silicon oxide wouldnot substantially change.

To form a silica aerogel coating having a porosity of 20-60% and arefractive index of 1.25-1.30, the ultrasonic radiation frequency ispreferably 10-30 kHz, the output is preferably 300-900 W, and theultrasonic treatment time is preferably 5-120 minutes.

A dispersing medium may be added to provide the silica-containing solwith appropriate concentration and fluidity. The dispersing medium maybe added before the ultrasonic treatment, or after conducting theultrasonic treatment to some extent. A mass ratio of the organicallymodified silicon oxide to the dispersing medium is preferably 0.1-20%.When the mass ratio of the organically modified silicon oxide to thedispersing medium is outside the range of 0.1-20%, a uniform thin layercannot be formed easily.

The use of a sol containing silicon oxide colloid particles havingnearly single dispersion can form an organically modified silica aerogellayer with small porosity. On the other hand, the use of a solcontaining largely agglomerated colloid particles can form a silicaaerogel layer with large porosity. Thus, the ultrasonic treatment timeinfluences the porosity of the silica aerogel coating. The coating ofthe sol ultrasonic-treated for 5-120 minutes can provide the organicallymodified silica aerogel layer with a porosity of 20-60%.

(ii) Preparation of Ultraviolet-Curable Resin Solution

The ultraviolet-curable resin functioning as a binder for theorganically modified silica preferably has compatibility with adispersion of the organically modified silica. As long as solvents candissolve the ultraviolet-curable resin and are compatible with theorganically modified silica dispersion, they are not restricted.Accordingly, they may be properly selected from those described above asthe substituting dispersion media for the organically modified silicadispersion.

The ultraviolet-curable resin has a refractive index of preferably 1.5or less, more preferably 1.3-1.4 after curing. Using anultraviolet-curable resin having a refractive index of 1.5 or less aftercuring, a silica aerogel coating having a refractive index of 1.2-1.3can be formed. Ultraviolet-curable, amorphous fluororesins preferablyhave a refractive index of 1.5 or less and excellent transparency.Specific examples of the ultraviolet-curable, amorphous fluororesinsinclude fluoroolefin copolymers, fluorine-containing cycloaliphaticpolymers, fluoroacrylate copolymers, etc.

An example of the fluoroolefin copolymer comprises 37-48% by mass oftetrafluoroethylene, 15-35% by mass of vinylidene fluoride, and 26-44%by mass of hexafluoropropylene.

Polymers having a fluorine-containing cycloaliphatic structure includethose obtained by polymerizing monomers having a fluorine-containingcycloaliphatic structure, and those obtained by the ring-formingpolymerization of fluorine-containing monomers having at least twopolymerizable double bonds. The polymers obtained by the polymerizationof monomers having a fluorine-containing ring structure are described inJP 63-18964 B, etc. They are obtained by the homo-polymerization ofmonomers having a fluorine-containing ring structure, such asperfluoro(2,2-dimethyl-1,3-dioxole), etc., or by their copolymerizationwith radically polymerizable monomers such as tetrafluoroethylene, etc.

The polymers obtained by the ring-forming polymerization offluorine-containing monomers having at least two polymerizable doublebonds are described in JP 63-238111 A, JP 63-238115 A, etc. They areobtained by the ring-forming polymerization of monomers such asperfluoro(allyl vinyl ether), perfluoro(butenyl vinyl ether), etc., orby their copolymerization with radically polymerizable monomers such astetrafluoroethylene, etc. Examples of the copolymers include thoseobtained by the copolymerization of monomers having afluorine-containing ring structure, such asperfluoro(2,2-dimethyl-1,3-dioxole), etc., with fluorine-containingmonomers having at least two polymerizable double bonds, such asperfluoro(allyl vinyl ether), perfluoro(butenyl vinyl ether), etc.

The binder may be made of a resin other than the fluororesin, or acombination of the fluororesin and the other resin. The resins otherthan the fluororesin may be acrylic resins, silicone resins, epoxyresins or urethane resins.

(iii) Preparation of Coating Liquid

The coating liquid comprises the organically modified silica, one ormore ultraviolet-curable resins, and a photo-polymerization initiator.The coating liquid can be obtained by (a) mixing a dispersion containingthe organically modified silica with a solution containing theultraviolet-curable resin and the photo-polymerization initiator, (b)mixing a dispersion containing the organically modified silica and thephoto-polymerization initiator with a solution containing theultraviolet-curable resin, (c) mixing a dispersion containing theorganically modified silica and the photo-polymerization initiator witha solution containing the ultraviolet-curable resin and thephoto-polymerization initiator, or (d) adding the photo-polymerizationinitiator after a dispersion containing the organically modified silicaand a solution containing the ultraviolet-curable resin are mixed. Thepercentage of the organically modified silica in the dispersion beforemixing is preferably 0.1-20% by mass per the dispersing medium asdescribed above. When the binder is a fluoroolefin copolymer, theconcentration of the copolymer is preferably 0.5-2.0% by mass.

The dispersion of organically modified silica and theultraviolet-curable resin solution are mixed preferably such that avolume ratio of the organically modified silica to theultraviolet-curable resin is 9:1-1:9 in the coating liquid. When thevolume ratio of the ultraviolet-curable resin is more than 90% in thecoating liquid, the pores of the silica aerogel are filled with theresin, resulting in the silica aerogel coating with too high arefractive index. When the volume ratio of the ultraviolet-curable resinis less than 10%, a binder ratio is too low to provide the silicaaerogel coating with toughness.

The photo-polymerization initiator is added to such an extent that theultraviolet-curable resin, or the ultraviolet-curable resin and theunsaturated groups of the organically modified silica can bepolymerized, in an ultraviolet irradiation step described later. Thephoto-polymerization initiator may be added to the ultraviolet-curableresin solution and/or the dispersion of organically modified silica inadvance, or after they are mixed. The amount of the photo-polymerizationinitiator is preferably 1-15% by mass per the coating liquid on a solidbasis.

Specific examples of the photo-polymerization initiators include benzoinand its derivatives such as benzoin methyl ether, benzoin isopropylether, benzoin isobutyl ether, etc.; benzyl derivatives such as benzyldimethyl ketal, etc.; alkyl phenones such as acetophenone,2,2-dimethoxy-2-phenylacetophenone, 2,2-diethoxy2-phenylacetophenone,1,1-dichloracetophenone, 1-hydroxycyclohexyl phenyl ketone,2-methyl-1-[4-(methylthio)phenyl]-2-morpholinopropane-1-on, etc.;anthraquinone and its derivatives such as 2-methylanthraquinone,2-chloroanthraquinone, 2-ethylanthraquinone, 2-t-butylanthraquinone,etc.; thioxanthone and its derivatives such as 2,4-dimethylthioxanthone,2-chlorothioxanthone, etc.; benzophenone and its derivatives such asN,N-dimethylamino benzophenone, etc.

(iv) Coating

Examples of wet-coating methods include a spray-coating method, aspin-coating method, a dip-coating method, a flow-coating method and abar-coating method. The preferred coating method is a spray-coatingmethod, which can form a layer with uniform thickness. When the coatingliquid is applied to a substrate, a dispersing medium is evaporated toform a layer composed of the organically modified silica, theultraviolet-curable resin and the photo-polymerization initiator.

(v) Drying

Because the coating liquid contains a volatile solvent, it may bespontaneously dried, but its drying may be accelerated by heating at50-100° C. Although the organically modified silica aerogel layer has aporosity reduced by the shrinkage of the gel due to capillary pressureduring the evaporation of the dispersing medium, the porosity isrecovered by a springback phenomenon after the completion ofevaporation. Thus, the porosity of the dried, organically modifiedsilica aerogel layer is substantially as large as the original one ofthe gel network. The shrinkage of a silica gel network and thespringback phenomenon are described in U.S. Pat. No. 5,948,482 indetail.

(vi) Ultraviolet Irradiation

Ultraviolet rays are irradiated to the coating to polymerize theultraviolet-curable resin, or the ultraviolet-curable resin and theunsaturated groups of the organically modified silica. Using anultraviolet irradiation apparatus, the coating is preferably subjectedto ultraviolet irradiation at about 50-10000 mJ/cm². The ultravioletirradiation time is preferably about 1-30 seconds when the silicaaerogel coating is as thick as about 10-2000 nm, although variabledepending on the coating thickness.

(vii) Baking

The coating is preferably baked at 50-150° C. The baking removes asolvent from the layer and a hydroxyl group, etc. from the surface,thereby strengthening the coating. Because decomposition does notsubstantially occur at a baking temperature of about 50-150° C., thebaked silica aerogel coating has a cured resin formed by thepolymerization of the ultraviolet-curable resin or theultraviolet-curable resin and the unsaturated groups of the organicallymodified silica.

(4) Substrate

The refractive index of the substrate 3 is 1.60-1.95, preferably1.65-1.80, in a wavelength range of 400-700 nm. Because the firstanti-reflection coating 1 exhibits an excellent anti-reflection functionto a substrate 3 having a refractive index of 1.60-1.95, it is effectiveto reduce the size of an exchange lens unit. Specific examples ofmaterials for the substrate 3 include optical glass such as BaSF2, SF5,LaF2, LaSF09, LaSF01, LaSF016, etc.

(5) Reflectance

The first anti-reflection coating 1 formed on the substrate 3 hasreflectance of preferably 0.5% or less, more preferably 0.4% or less, toincident light at 0° in a wavelength range of 450-600 nm, andreflectance of preferably 1.0% or less, more preferably 0.95% or less,to incident light at 300 in a wavelength range of 400-650 nm.

[2] Second Anti-Reflection Coating

The second anti-reflection coating 1 according to another embodiment ofthe present invention is the same as the first anti-reflection coatingexcept for the refractive index and optical thickness of each layer.Namely, in a wavelength range of 400-700 nm,

-   -   the substrate has a refractive index of 1.50-1.70,    -   the first layer 11 has an optical thickness of 24.5-200.0 nm,    -   the second layer 12 has a refractive index of 1.93-2.25 and an        optical thickness of 24.5-50.5 nm,    -   the third layer 13 has a refractive index of 1.33-1.50 and an        optical thickness of 14.0-30.0 nm,    -   the fourth layer 14 has a refractive index of 2.00-2.16 and an        optical thickness of 131.5-200.5 nm,    -   the fifth layer 15 has a refractive index of 1.33-1.50 and an        optical thickness of 20.0-31.5 nm,    -   the sixth layer 16 has a refractive index of 2.04-2.17 and an        optical thickness of 50.0-62.5 nm, and    -   the seventh layer 17 has an optical thickness of 122.5-142.5 nm.

The refractive index of the first layer 11 is preferably 1.57-1.72, morepreferably 1.63-1.67. The first layer 11 preferably has an opticalthickness of 30.0-200.0 nm. The second layer 12 preferably has anoptical thickness of 30.0-45.0 nm and a refractive index of 2.02-2.20.The third layer 13 preferably has an optical thickness of 20.0-28.0 nmand a refractive index of 1.36-1.47. The fourth layer 14 preferably hasan optical thickness of 150.0-183.0 nm and a refractive index of2.00-2.15. The fifth layer 15 preferably has an optical thickness of22.0-30.0 nm and a refractive index of 1.36-1.47. The sixth layer 16preferably has an optical thickness of 50.0-60.0 nm and a refractiveindex of 2.05-2.15. The refractive index of the seventh layer 17 ispreferably 1.23-1.32, more preferably 1.26-1.29. The seventh layerpreferably has an optical thickness of 125.0-138.0 nm.

The refractive index of the substrate 3, on which the secondanti-reflection coating 1 is formed, is 1.50-1.70, preferably 1.51-1.60,in a wavelength range of 400-700 nm. Because the second anti-reflectioncoating 1 has an excellent anti-reflection function to the substrate 3having a refractive index of 1.50-1.70, it is effective to reduce thesize of an exchange lens unit. Specific examples of materials for thesubstrate 3 include optical glass such as LF5, BK7, BAK1, BAK2, K3,PSK2, SK4, SK5, SK7, SK11, SK12, SK14, SK15, SK16, SK18, KF3, SK6, SK8,BALF2, SSK5, LLF1, LLF2, LLF6, BAF10, BAF11, BAF12, F1, F5, F8, F16,SF2, SF7, KZF2, KZF5, LAK11, LAK12, etc.

The second anti-reflection coating 1 formed on the substrate 3 hasreflectance of preferably 0.3% or less, more preferably 0.26% or less,to incident light at 0° in a wavelength range of 450-600 nm.

[3] Fluororesin Layer

As shown in FIG. 2, a fluororesin layer 18 having water repellency orwater/oil repellency (simply called “water/oil repellency”) may beformed on the seventh layer 17. The fluororesins are not particularlyrestricted as long as they are colorless and transparent, but they arepreferably fluorine-containing polymers, fluorinated pitch, ororganic-inorganic hybrid polymers.

The fluorine-containing polymers are preferably fluorine-containingolefinic polymers or copolymers, such as polytetrafluoroethylene (PTFE),tetraethylene-hexafluoropropylene copolymers (PFEP),ethylene-tetrafluoroethylene copolymers (PETFE),tetrafluoroethylene-perfluoroalkyl vinyl ether copolymers (PFA),ethylene-chlorotrifluoroethylene copolymers (PECTFE),tetrafluoroethylene-hexafluoropropylene-perfluoroalkyl vinyl ethercopolymers (PEPE), polychlorotrifluoroethylene (PCTFE), polyvinylidenefluoride (PVDF), polyvinyl fluoride (PVF), etc. Commercially availablefluorine-containing polymers may be, for instance, “OPSTAR” availablefrom JSR Corporation, “CYTOP” available from Asahi Glass Co., Ltd.

The fluorine-containing organic-inorganic hybrid polymers may be organicsilicon polymers having fluorocarbon groups, which may be polymersobtained by the hydrolysis of fluorine-containing silane compounds. Thefluorine-containing silane compounds may be compounds represented by thefollowing formula (15):

CF₃(CF₂)_(a)(CH₂)₂SiR_(b)X_(c)   (15),

wherein R is an alkyl group, X is an alkoxyl group or a halogen atom, ais an integer of 0-7, b is an integer of 0-2, c is an integer of 1-3,and b+c=3. Specific examples of the compounds represented by the formula(15) include CF₃(CH₂)₂Si(OCH₃)₃, CF₃(CH₂)₂SiCl₃,CF₃(CF₂)₅(CH₂)₂Si(OCH₃)₃, CF₃(CF₂)₅(CH₂)₂SiCl₃,CF₃(CF₂)₇(CH₂)₂Si(OCH₃)₃, CF₃(CF₂)₇(CH₂)₂SiCl₃,CF₃(CF₂)₇(CH₂)₃SiCH₃(OCH₃)₂, CF₃(CF₂)₇(CH₂)₂SiCH₃Cl₂, etc. Examples ofcommercially available organic silicon polymers include Novec EGC-1720available from Sumitomo 3M Ltd., XC98-B2472 available from GE ToshibaSilicone Co., Ltd., X71-130 available from Shin-Etsu Chemical Co., Ltd.,etc.

The fluororesin layer 18 is as thick as preferably 0.4-100 nm. When thethickness of the fluororesin layer is less than 0.4 nm, sufficientwater/oil repellency cannot be obtained. On the other hand, with thefluororesin layer thicker than 100 nm, the anti-reflection coating hasdeteriorated transparency and changed optical properties. The refractiveindex of the fluororesin layer is preferably 1.5 or less, morepreferably 1.45 or less. Although the fluororesin layer may be formed bya vacuum vapor deposition method, it is preferably formed by a wetmethod such as a sol-gel method.

The present invention will be explained in further detail by Examplesbelow without intention of restricting the present invention thereto.

EXAMPLE 1

An anti-reflection coating 1 having the layer structure shown in Table 1was produced by the following steps. The refractive index of each layeris to light having a wavelength of 550 nm.

[1] Formation of First to Sixth Layers

Using the apparatus shown in FIG. 13, the first to sixth layers shown inTable 1 were formed on an optical lens of LaSF01 by an electron-beamvacuum vapor deposition method at an initial degree of vacuum of1.2×10⁻⁵ Torr and a substrate temperature of 230° C.

[2] Formation of Seventh Layer

A silica aerogel layer as the seventh layer was formed by a sol-gelmethod comprising the following steps.

(2-i) Preparation of Wet Silica Gel

After 5.21 g of tetraethoxysilane and 4.38 g of ethanol were mixed, 0.4g of hydrochloric acid (0.01 N) was added, and stirring was conductedfor 90 minutes. With 44.3 g of ethanol and 0.5 g of ammonia water (0.02N) added, stirring was conducted for 46 hours, and then this mixedsolution was aged at 60° C. for 46 hours to obtain a wet silica gel.

(2-ii) Preparation of Dispersion of Organically Modified Silica

The wet silica gel was mixed with ethanol, vibrated for 10 hours, andthen decanted to remove unreacted materials, and to change thewet-gel-dispersing medium to ethanol. With methyl isobutyl ketone (MIBK)added, the ethanol-dispersed wet gel was vibrated for 10 hours, and thendecanted to change the dispersing medium from ethanol to MIBK.

The wet silica gel was mixed with a solution of trimethylchlorosilane inMIBK (concentration: 5% by volume) and stirred for 30 hours toorganically modify silicon oxide at ends. The resultant organicallymodified wet silica gel was washed with MIBK by vibration for 24 hoursand decantation.

MIBK was added to the organically modified wet silica gel to aconcentration of 3% by mass, and ultrasonic irradiation (20 kHz, 500 W)was conducted for 20 minutes to turn the wet silica gel to a sol-like,organically modified silica (dispersion of the organically modifiedsilica).

(2-iii) Preparation of Coating Liquid

The dispersion of the organically modified silica prepared in the step(2-ii) was mixed with a low-refractive-index, ultraviolet-curable resinsolution (“AR100” available from Daikin Industries, Ltd.) at a volumeratio of 9:1 to prepare a coating liquid containing the organicallymodified silica.

(2-iv) Spin-Coating

The coating liquid containing the organically modified silica wasapplied to the sixth layer by a spin-coating method, subjected toultraviolet irradiation at 1500 mJ/cm² using an ultraviolet irradiationapparatus available from Fusion Systems, and baked at 150° C. for 1 hourto form a silica aerogel coating, in which the ultraviolet-curable resinwas polymerized, and the silica sol was hydrolyzed and polycondensed tohave organically modified chains. The thickness of the silica aerogellayer is shown in Table 1.

TABLE 1 Refractive Optical No. Material Index Thickness (nm) SubstrateLaSF01 1.790 — First Layer Al₂O₃ 1.650 75.0 Second Layer Ta₂O₅ + Y₂O₃ +Pr₆O₁₁ 2.050 47.8 Third Layer MgF₂ 1.380 32.9 Fourth Layer Ta₂O₅ +Y₂O₃ + Pr₆O₁₁ 2.050 235.9 Fifth Layer MgF₂ 1.380 20.9 Sixth LayerTa₂O₅ + Y₂O₃ + Pr₆O₁₁ 2.050 37.9 Seventh Layer Silica Aerogel 1.270123.1 Medium Air 1.000 —

EXAMPLE 2

An anti-reflection coating having the layer structure shown in Table 2was formed in the same manner as in Example 1.

TABLE 2 Refractive Optical No. Material Index Thickness (nm) SubstrateLaSF01 1.790 — First Layer Al₂O₃ 1.650 85.0 Second Layer Ta₂O₅ + Y₂O₃ +Pr₆O₁₁ 2.050 40.0 Third Layer MgF₂ 1.380 28.0 Fourth Layer Ta₂O₅ +Y₂O₃ + Pr₆O₁₁ 2.050 215.0 Fifth Layer MgF₂ 1.380 25.0 Sixth LayerTa₂O₅ + Y₂O₃ + Pr₆O₁₁ 2.050 35.0 Seventh Layer Silica Aerogel 1.270115.0 Medium Air 1.000 —

EXAMPLE 3

An anti-reflection coating having the layer structure shown in Table 3was formed in the same manner as in Example 1.

TABLE 3 Refractive Optical No. Material Index Thickness (nm) SubstrateLaSF01 1.790 — First Layer Al₂O₃ 1.650 75.0 Second Layer Ta₂O₅ + Y₂O₃ +Pr₆O₁₁ 2.050 47.8 Third Layer MgF₂ 1.380 32.9 Fourth Layer Ta₂O₅ +Y₂O₃ + Pr₆O₁₁ 2.050 230.0 Fifth Layer MgF₂ 1.380 20.9 Sixth LayerTa₂O₅ + Y₂O₃ + Pr₆O₁₁ 2.050 37.9 Seventh Layer Silica Aerogel 1.270137.5 Medium Air 1.000 —

EXAMPLE 4

An anti-reflection coating having the layer structure shown in Table 4was formed in the same manner as in Example 1.

TABLE 4 Refractive Optical No. Material Index Thickness (nm) SubstrateLaSF01 1.790 — First Layer Al₂O₃ 1.650 100.0 Second Layer Ta₂O₅ + Y₂O₃ +Pr₆O₁₁ 2.050 37.5 Third Layer MgF₂ 1.380 37.5 Fourth Layer Ta₂O₅ +Y₂O₃ + Pr₆O₁₁ 2.050 245.0 Fifth Layer MgF₂ 1.380 18.8 Sixth LayerTa₂O₅ + Y₂O₃ + Pr₆O₁₁ 2.050 32.5 Seventh Layer Silica Aerogel 1.270123.1 Medium Air 1.000 —

EXAMPLE 5

An anti-reflection coating having the layer structure shown in Table 5was formed in the same manner as in Example 1.

TABLE 5 Refractive Optical No. Material Index Thickness (nm) SubstrateLaSF01 1.790 — First Layer Al₂O₃ 1.650 75.0 Second Layer ZrO₂ + TiO₂2.110 47.8 Third Layer SiO₂ 1.450 32.9 Fourth Layer ZrO₂ + TiO₂ 2.110235.9 Fifth Layer SiO₂ 1.450 20.9 Sixth Layer ZrO₂ + TiO₂ 2.110 37.9Seventh Layer Silica Aerogel 1.270 123.1 Medium Air 1.000 —

EXAMPLE 6

An anti-reflection coating having the first to seventh layers shown inTable 6 was formed on an optical lens of LaSF01 in the same manner as inExample 1, immersed in a coating liquid comprising 3 g of asilicone-type fluororesin (“X71-130” available from Shin-Etsu ChemicalCo., Ltd.) and 60 g of hydrofluoroether, lifted out of the coatingliquid at 300 mm/minute, and dried at room temperature for 24 hours toobtain a fluororesin layer.

TABLE 6 Refractive Optical No. Material Index Thickness (nm) SubstrateLaSF01 1.790 — First Layer Al₂O₃ 1.650 75.0 Second Layer Ta₂O₅ + Y₂O₃ +Pr₆O₁₁ 2.050 47.8 Third Layer MgF₂ 1.380 32.9 Fourth Layer Ta₂O₅ +Y₂O₃ + Pr₆O₁₁ 2.050 235.9 Fifth Layer MgF₂ 1.380 20.9 Sixth LayerTa₂O₅ + Y₂O₃ + Pr₆O₁₁ 2.050 37.9 Seventh Layer Silica Aerogel 1.270123.1 Eighth Layer Fluororesin Layer 1.400 35.0 Medium Air 1.000 —

EXAMPLE 7

An anti-reflection coating having the layer structure shown in Table 7was formed on an optical lens of LF5 in the same manner as in Example 1.

TABLE 7 Refractive Optical No. Material Index Thickness (nm) SubstrateLF5 1.584 — First Layer Al₂O₃ 1.650 81.1 Second Layer Ta₂O₅ + Y₂O₃ +Pr₆O₁₁ 2.050 39.7 Third Layer MgF₂ 1.380 23.1 Fourth Layer Ta₂O₅ +Y₂O₃ + Pr₆O₁₁ 2.050 157.7 Fifth Layer MgF₂ 1.380 25.0 Sixth LayerTa₂O₅ + Y₂O₃ + Pr₆O₁₁ 2.050 56.5 Seventh Layer Silica Aerogel 1.270131.3 Medium Air 1.000 —

EXAMPLE 8

An anti-reflection coating having the layer structure shown in Table 8was formed in the same manner as in Example 7.

TABLE 8 Refractive Optical No. Material Index Thickness (nm) SubstrateLF5 1.584 — First Layer Al₂O₃ 1.650 200.0 Second Layer Ta₂O₅ + Y₂O₃ +Pr₆O₁₁ 2.050 37.5 Third Layer MgF₂ 1.380 25.0 Fourth Layer Ta₂O₅ +Y₂O₃ + Pr₆O₁₁ 2.050 182.5 Fifth Layer MgF₂ 1.380 22.5 Sixth LayerTa₂O₅ + Y₂O₃ + Pr₆O₁₁ 2.050 50.0 Seventh Layer Silica Aerogel 1.270125.0 Medium Air 1.000 —

EXAMPLE 9

An anti-reflection coating having the layer structure shown in Table 9was formed in the same manner as in Example 7.

TABLE 9 Refractive Optical No. Material Index Thickness (nm) SubstrateBK7 1.518 — First Layer Al₂O₃ 1.650 81.1 Second Layer ZrO₂ + TiO₂ 2.11039.7 Third Layer SiO₂ 1.462 23.1 Fourth Layer ZrO₂ + TiO₂ 2.110 157.7Fifth Layer SiO₂ 1.462 25.0 Sixth Layer ZrO₂ + TiO₂ 2.110 56.5 SeventhLayer Silica Aerogel 1.270 131.3 Medium Air 1.000 —

EXAMPLE 10

An anti-reflection coating having the first to seventh layers shown inTable 10 was formed on an optical lens of LF5 in the same manner as inExample 7, and then the same fluororesin layer as in Example 6 wasformed.

TABLE 10 Refractive Optical No. Material Index Thickness (nm) SubstrateLF5 1.584 — First Layer Al₂O₃ 1.650 81.1 Second Layer Ta₂O₅ + Y₂O₃ +Pr₆O₁₁ 2.050 39.7 Third Layer MgF₂ 1.380 23.1 Fourth Layer Ta₂O₅ +Y₂O₃ + Pr₆O₁₁ 2.050 157.7 Fifth Layer MgF₂ 1.380 25.0 Sixth LayerTa₂O₅ + Y₂O₃ + Pr₆O₁₁ 2.050 56.5 Seventh Layer Silica Aerogel 1.270131.3 Eighth Layer Fluororesin Layer 1.400 35.0 Medium Air 1.000 —

COMPARATIVE EXAMPLE 1

An anti-reflection coating having the layer structure shown in Table 11was formed in the same manner as in Example 1 except for using MgF₂ forthe seventh layer.

TABLE 11 Refractive Optical No. Material Index Thickness (nm) SubstrateLaSF01 1.790 — First Layer Al₂O₃ 1.650 171.75 Second Layer ZrO₂ + TiO₂2.110 25.50 Third Layer MgF₂ 1.380 36.00 Fourth Layer ZrO₂ + TiO₂ 2.110108.25 Fifth Layer MgF₂ 1.380 23.25 Sixth Layer ZrO₂ + TiO₂ 2.110 89.00Seventh Layer MgF₂ 1.380 134.50 Medium Air 1.000 —

COMPARATIVE EXAMPLE 2

An anti-reflection coating having the layer structure shown in Table 12was formed in the same manner as in Example 7 except for using MgF₂ forthe seventh layer.

TABLE 12 Refractive Optical No. Material Index Thickness (nm) SubstrateLF5 1.584 — First Layer Al₂O₃ 1.651 36.38 Second Layer Ta₂O₅ 2.097 24.25Third Layer MgF₂ 1.389 37.34 Fourth Layer Ta₂O₅ 2.097 102.74 Fifth LayerMgF₂ 1.389 25.09 Sixth Layer Ta₂O₅ 2.097 82.84 Seventh Layer MgF₂ 1.389134.32 Medium Air 1.000 —

FIGS. 3-8 show the spectral reflectance characteristics of theanti-reflection-coated optical lenses of Examples 1-5 and ComparativeExample 1, which were measured by light in a wavelength range of 350-850nm at incident angles of 0° and 30°. A medium in contact with theoutermost layer was air. As shown in FIGS. 3-7, Examples 1-5 exhibitedlow reflectance to visible light in a wavelength range of 400-700 nm atany incident angles of 0° and 30°. On the other hand, as shown in FIG.8, Comparative Example 1 exhibited reflectance higher than 0.5% near awavelength of 560 nm when the incident angle was 0°.

FIGS. 9-12 show the spectral reflectance characteristics of theanti-reflection-coated optical lenses of Examples 7-9 and ComparativeExample 2, which were measured by light in a wavelength range of 350-850nm at an incident angle of 0°. A medium in contact with the outermostlayer was air. As shown in FIGS. 9-11, Example 7-9 exhibited lowreflectance in a visible wavelength range of 400-700 nm. On the otherhand, as shown in FIG. 12, Comparative Example 2 exhibited reflectancehigher than 0.3% near a wavelength of 480 nm.

Photographs taken with the optical lenses of Examples 1-10 had no flareand ghost, while those taken with the optical lenses of ComparativeExamples 1 and 2 had flare and ghost.

EFFECT OF THE INVENTION

Because the anti-reflection coating of the present invention comprisesan alumina-based, innermost layer and a porous, silica-based, outermostlayer, as well as intermediate layers having controlled refractiveindices and optical thickness, it has an excellent anti-reflectioneffect to visible light in a wavelength range of 400-700 nm even with asfew as 7 layers in total, without suffering tarnish. The anti-reflectioncoating of the present invention having such feature is suitable forexchange lens units for single-lens reflex cameras, etc., providingphotographs free from flare and ghost.

The present disclosure relates to subject matter contained in JapanesePatent Application No. 2007-337906 (filed on Dec. 27, 2007) and JapanesePatent Application No. 2008-003644 (filed on Jan. 10, 2008), which areexpressly incorporated herein by reference in their entireties.

1. An anti-reflection coating comprising first to seventh layers formedon a substrate in this order, said first layer being an alumina-basedlayer, said seventh layer being a porous, silica-based layer, and in awavelength range of 400-700 nm, said substrate having a refractive indexof 1.60-1.95, said first layer having an optical thickness of 37.5-112.5nm, said second layer having a refractive index of 1.95-2.25 and anoptical thickness of 35.5-60.0 nm, said third layer having a refractiveindex of 1.35-1.50 and an optical thickness of 24.5-41.5 nm, said fourthlayer having a refractive index of 1.95-2.25 and an optical thickness of210.5-250.0 nm, said fifth layer having a refractive index of 1.35-1.50and an optical thickness of 12.5-32.5 nm, said sixth layer having arefractive index of 1.95-2.25 and an optical thickness of 27.5-45.0 nm,and said seventh layer having an optical thickness of 108.0-138.0 nm. 2.The anti-reflection coating according to claim 1, wherein said secondlayer, said fourth layer and said sixth layer are made of at least onematerial selected from the group consisting of Ta₂O₅, ZrO₂, HfO₂, CeO₂,SnO₂, In₂O₃ and ZnO, and said third layer and said fifth layer are madeof MgF₂ and/or SiO₂.
 3. The anti-reflection coating according to claim1, wherein said first layer has a refractive index of 1.59-1.69.
 4. Theanti-reflection coating according to claim 1, wherein said seventh layerhaving a refractive index of 1.25-1.30.
 5. The anti-reflection coatingaccording to claim 1, which has reflectance of 0.5% or less to incidentlight at 0° in a wavelength range of 450-600 nm, and reflectance of 1.0%or less to incident light at 300 in a wavelength range of 400-650 nm. 6.An anti-reflection coating comprising first to seventh layers formed ona substrate in this order, said first layer being an alumina-basedlayer, said seventh layer being a porous, silica-based layer, and in awavelength range of 400-700 nm, said substrate having a refractive indexof 1.50-1.70, said first layer having an optical thickness of 24.5-200.0nm, said second layer having a refractive index of 1.93-2.25 and anoptical thickness of 24.5-50.5 nm, said third layer having a refractiveindex of 1.33-1.50 and an optical thickness of 14.0-30.0 nm, said fourthlayer having a refractive index of 2.00-2.16 and an optical thickness of131.5-200.5 nm, said fifth layer having a refractive index of 1.33-1.50and an optical thickness of 20.0-31.5 nm, said sixth layer having arefractive index of 2.04-2.17 and an optical thickness of 50.0-62.5 nm,and said seventh layer having an optical thickness of 122.5-142.5 nm. 7.The anti-reflection coating according to claim 6, wherein said secondlayer, said fourth layer and said sixth layer are made of at least onematerial selected from the group consisting of Ta₂O₅, ZrO₂, HfO₂, CeO₂,SnO₂, In₂O₃ and ZnO, and said third layer and said fifth layer are madeof MgF₂ and/or SiO₂.
 8. The anti-reflection coating according to claim6, wherein said first layer has a refractive index of 1.57-1.72.
 9. Theanti-reflection coating according to claim 6, wherein said seventh layerhas a refractive index of 1.23-1.32.
 10. The anti-reflection coatingaccording to claim 6, which has reflectance of 0.3% or less to incidentlight at 0° in a wavelength range of 450-600 nm.
 11. The anti-reflectioncoating according to claim 1, wherein said seventh layer is a silicaaerogel layer.
 12. The anti-reflection coating according to claim 1,wherein a fluororesin layer as thick as 0.4-100 nm having waterrepellency or water/oil repellency is formed on said seventh layer. 13.The anti-reflection coating according to claim 1, wherein said seventhlayer is formed by a sol-gel method.
 14. An optical member having theanti-reflection coating recited in claim
 1. 15. An exchange lens unitcomprising the optical member recited in claim
 14. 16. An imaging devicecomprising the optical member recited in claim
 14. 17. Theanti-reflection coating according to claim 6, wherein said seventh layeris a silica aerogel layer.
 18. The anti-reflection coating according toclaim 6, wherein a fluororesin layer as thick as 0.4-100 nm having waterrepellency or water/oil repellency is formed on said seventh layer. 19.The anti-reflection coating according to claim 6, wherein said seventhlayer is formed by a sol-gel method.
 20. An optical member having theanti-reflection coating recited in claim
 6. 21. An exchange lens unitcomprising the optical member recited in claim
 20. 22. An imaging devicecomprising the optical member recited in claim 20.